The motor armature shaft for an electric motor may consist of a hollow tubular member to allow another shaft to be passed inside the bore of the armature shaft, for instance, as disclosed in Japanese patent laid-open (kokai) publication No. 6-39473 by the same Applicant. Such a motor armature shaft is normally formed with a serration on an outer circumferential surface thereof by placing a tubular member in a bore of a forming die formed with a serration on a part of an inner surface thereof, and squeezing a forming punch axially through the tubular member so that the inner surface of the tubular member may be formed into a prescribed configuration while the serration is formed on the outer circumferential surface of the tubular member. An armature core can be therefore securely attached to the outer circumferential surface of the tubular member via the serration.
It is also known, as disclosed in Japanese patent laid-open (kokai) publication No. 5-260704, to loosely fit an armature core, having a serration on an inner surface of a bore defined therein, on a tubular armature shaft, and squeeze a forming punch axially through the inner bore of the armature shaft so as to force the armature shaft into an outward plastic deformation, and integrally join the armature core and the armature shaft by bringing the material of the armature shaft in close contact with the serration on the inner circumferential surface of the bore of the armature core. According to this previous proposal, the outer diameter of the armature shaft and the inner diameter of the armature core may be defined with a certain dimensional tolerance, and the dimensional control during the manufacturing process can be simplified.
However, according to the armature shaft having the above described structure, because the part of the armature shaft, which receives the part of the maximum outer diameter of the other shaft, must have a larger inner diameter than this maximum outer diameter, when an attempt is made to minimize the diameter of the armature shaft carrying the armature core insofar as necessary to ensure a sufficient cross sectional area for the magnetic path, the wall thickness of the axially, or axial, end portion of the armature shaft for mounting a bearing is reduced as a result. The reduction in the wall thickness of this part of the armature shaft may cause this part to be inwardly deformed as a nut for securing the bearing is fastened, and the nut may not be able to achieve a required fastening force.
If the wall thickness of the tubular shaft is increased so that a desired mechanical strength may be achieved at the axial end thereof, the outer diameter of the armature shaft necessarily increases, and the outer diameter of the armature core has to be increased so as to ensure a sufficient magnetic path. This leads to an increase in the outer diameter and the weight of the motor.
Thus, a primary task of the present invention is to provide a motor armature arrangement for an electric motor which allows a sufficient mechanical strength to be achieved while minimizing the outer diameter of the motor.